A photovoltaic conversion element, a light emitting element, a capacitor, and the like are elements employing a phenomenon taking place when two kinds of materials are brought into contact with each other or disposed in close proximity to each other. For example, the photovoltaic conversion element and the light emitting element employ a phenomenon taking place on the contact interface between a p-type semiconductor and an n-type semiconductor that light is generated by generation of electron-hole pairs or recombination of electrons and holes according to an amount of received light. The capacitor employs polarization of a dielectric material sandwiched between a pair of electrodes.
It is possible for these elements to enhance the properties of the elements by increasing the contact area or the opposing area of two kinds of materials. For example, by increasing the area of the interface between the p-type semiconductor and the n-type semiconductor, an amount of electron-hole pairs to be generated, that is, the magnitude of a photoelectric current (photoelectromotive force) can be increased in the photovoltaic conversion element, and an amount of light to be generated can be increased in the light emitting element. Also, by increasing the contact (opposing) area of the dielectric material and the electrodes, the electrostatic capacity can be increased in the capacitor.
In order to increase the contact area or the opposing area of two kinds of materials, an attempt is being made by using an aggregate of needle-like crystals having a large surface area (specific surface area) for these elements.
For example, Patent Document 1 specified below discloses a photovoltaic conversion element including a transparent electrode, needle-like crystals forming one charge transporting layers and formed on the transparent electrode, and the other charge transporting layer provided to come into contact with (to oppose) the needle-like crystals.
Also, Patent Document 2 specified below discloses a capacitor including a flat storage node made of polycrystalline silicon, plural needle-like crystals made of a material having the conducting property, such as germanium, and formed on the storage node, and an insulating film (dielectric material) for capacitor made of silicon oxide and provided to cover the surface of the needle-like crystals.
Patent Document 3 specified below discloses a method for producing an array of needle-like crystals by heating a substrate (base) in a solution containing amine, such as hexamethylene tetramine, as well as polyethyleneimine and zinc ions, and a device using the array of needle-like crystals produced by this producing method. As such a device, a dye sensitizing photovoltaic cell (a dye sensitizing photovoltaic cell using the array of needle-like crystals as a semiconductor having the p-type physical property) and a light emitting diode are disclosed therein. Patent Document 3 specified below further discloses a current versus voltage characteristic of an FET (Field Effect Transistor) using an array of zinc oxide needle-like crystals.
According to the producing method of Patent Document 3 specified below, it is impossible to cover the base with the array densely (substantially completely) in a region on the base side. Accordingly, as means for preventing current leakage from a portion of the base exposed through the array, an electronic block layer (insulator) is provided in clearances among needle-like crystals forming the array when the device is fabricated.
Also, Non-Patent Document 3 specified below discloses a method for producing a light emitting diode using an array of zinc oxide needle-like crystals and the properties thereof. An array of zinc oxide needle-like crystals is formed by means of electro-deposition. However, because it is impossible to cover the base with the array densely (substantially completely) in a region on the base side, an insulator is provided in clearances among needle-like crystals to prevent current leakage.
In Patent Document 3 and Non-Patent Document 3 specified below, insulating polymer, such as polymethyl methacrylate and polystyrene or the like, is used as an insulator to prevent the generation of a leak current. The insulator in each is formed to cover the zinc oxide needle-like crystals first, and thence a portion present at the tip ends of zinc oxide needle-like crystals is removed by means of UV irradiation, plasma irradiation, or the like, so that the insulator is left in clearances among the zinc oxide needle-like crystals.
Non-Patent Document 4 discloses a method for producing an array of zinc oxide needle-like crystals on a foundation layer made of zinc oxide by means of electroless plating. According to this method, a layer used as the foundation layer is obtained by applying a 2-methoxymethanol solution, in which zinc acetate dihydrate and monoethanolamine are dissolved, on the base followed by drying at 60° C. for 24 hours. The thickness of the foundation layer is in the order of 100 nm. In a case where the concentration of zinc in the plating solution is 0.01 mol/l, an array of zinc oxide needle-like crystals is obtained by regulating a pH of the plating solution during plating to 9 to 13.
Non-Patent Document 5 specified below discloses a method in which a zinc oxide thin film is formed on a glass substrate by means of sputtering and an array of zinc oxide needle-like crystals is formed using this zinc oxide thin film as the seeds. It is said that this method makes it possible to obtain needle-like crystals aligned in orientation in comparison with needle-like crystals obtained by general liquid phase growth.    Patent Document 1: Japanese Unexamined Patent Publication No. 2002-356400    Patent Document 2: Japanese Unexamined Patent Publication No. 6-252360    Patent Document 3: U.S. Patent Application Publication No. US2005/0009224 A1    Non-Patent Document 1: Michael H. Huang and eight others, “Room-Temperature Ultraviolet Nanowire Nanolasers”, SCIENCE vol. 292 p. 1897-1899 (8 Jun., 2001)    Non-Patent Document 2: Masanobu Izaki and one other, “Transparent zinc oxide films prepared by electrochemical reaction”, Appl. Phys. Lett. 68(17), (22 Apr., 1996)    Non-Patent Document 3: R. Konenkamp and two others, “Ultraviolet Electroluminescence from ZnO/Polymer Heterojunction Light-Emitting Diodes”, Nano Letters, vol. 5 p. 2005 (17 Sep., 2005)    Non-Patent Document 4: Satoshi Yamabi and one other, “Growth conditions for wurtzite zinc oxide films in aqueous solutions”, J. Mater. Chem., 12, 3773, (2002)    Non-Patent Document 5: R. B. Peterson and two others, “Epitaxial Chemical Deposition of ZnO Nanocolumns from NaOH Solutions”, Langmuir, 20, 5114, (2004)    Non-Patent Document 6: edited by the Surface Science Society of Japan, Kaitei-ban, Hyoumen Kagaku no Kiso to Ouyou, NTS Inc.    Non-Patent Document 7: A. Fujishima and one other, “Electrochemical Photolysis of Water at a Semiconductor Electrode” Nature, 238, 37 (1972)